The present invention relates to the field of optical fiber Mach Zehnder interferometers. More specifically, the present invention is directed to an optical fiber Mach Zehnder interferometer having miniature bends in one or more of its interfering arms.
It is well known to form an unbalanced Mach Zehnder interferometer with two couplers across a pair of straight parallel fibers. FIG. 1 depicts a typical fiber optic Mach Zehnder interferometer 10 of the prior art. Interferometer 10 includes a first and second elongate optical fiber 12 and 14 which are optically-coupled at a first and second optical coupler 22 and 24. Fibers 12 and 14 each define an interfering arm 16 and 18, respectively, extending between couplers 22 and 24. Interfering arms 16 and 18 are arranged to have unequal optical lengths by known methods such as having one of the interfering arms longer than the other, utilizing fibers with different propagation constants, or a combination of these approaches. The magnitude of the imbalance in the optical path length of these phase-sensitive interfering arms determines the wavelength sensitivity of the interferometer.
If the two phase-sensitive segments are perfectly balanced in optical path length, then the light launched into fiber 12 couples with fiber 14 at coupler 22 and interferes at coupler 24 so that all of the light emerges from fiber 12 past coupler 24. If the phase-sensitive segments 16 and 18 are unbalanced, then the light will emerge in various ratios from fibers 12 and 14 past coupler 24 depending on the wavelength of the light and the magnitude of the path-length imbalance. Typically the light oscillates between the top and bottom fiber as the wavelength is increased. The larger the path-length imbalance, the smaller the wavelength change which results in the transfer of the light from one fiber to the other.
It is difficult, however, to create fiber segments with physically different lengths without introducing loss or making the construction tedious and difficult. Moreover, as shown in FIG. 1, the resulting structure of interferometer 10 has fibers emerging in opposing directions so that additional space has to be allowed to accommodate the bend radius of the fibers at both ends during subsequent packaging.
It is also known, however, to impart miniature bends to an optical fiber so as to reduce its minimum radius of curvature. As disclosed in the commonly-assigned U.S. Pat. No. 5,138,676 to Stowe et al., which is hereby incorporated by reference in the present application, the transmissive optical core of an optical fiber may be drawn to a significantly reduced diameter. The reduced core may be bent and then annealed to provide a bend in the optical fiber which exhibits very low optical power loss. Miniature bends may be formed having a radius of less than 0.5 millimeters without high attenuation and with low internal stress. For example, the technology allows low-loss 180xc2x0 bends to be formed in a package less than 2.0 millimeters in diameter and 8.0 millimeters long. Such low-loss bends may be formed in both singlemode and multimode fibers.
The diameter reduction is typically achieved by tapering the fiber, chemically removing some of the cladding glass, or a combination of these techniques. For a singlemode device, the fiber is processed so that the fundamental mode of the original fiber evolves adiabatically into the fundamental mode of the modified fiber to avoid light loss. The bend may be housed in a variety of packages provided no material comes into contact with the fiber in the processed region.
There is therefore a need for an optical Mach Zehnder interferometer which incorporates miniature bends in its component optical fibers so as to minimize interferometer size, thermal sensitivity, and vibration sensitivity.
The present invention provides an optical fiber Mach Zehnder Interferometer having a first and second elongate optical fiber, each having a core and a cladding, and first and second couplers wherein the cladding of said first optical fiber is coupled to the cladding of the second optical fiber. The Mach Zehnder Interferometer of the present invention further includes a first elongate interfering arm formed by that portion of the first optical fiber extending between the first and second couplers, and a second elongate interfering arm comprising the second optical fiber extending between the first and second couplers. Additionally, the first interfering arm includes at least one miniature bend formed therein. The second interfering arm may also include a miniature bend formed therein. In one embodiment of the present invention, the miniature bend of the first interfering arm is nested within the second interfering arm.
Moreover, the Mach Zehnder Interferometer of the present invention may incorporate either prepackaged miniature bends or unpacked miniature bends. The interfering arms may be adhesively tacked to a supporting substrate with either an adhesive epoxy or, when employing unpackaged miniature bends, an adhesive gel. The Mach Zehnder Interferometer of the present invention may further exhibit reduced thermal sensitivity by selecting optical fibers having different thermal expansion coefficients to account for different total length changes due to temperature between two interfering arms of unequal length.